The present invention relates to clear coat systems based on acid epoxy-isocyanate chemistry wherein one or more of the components can be modified such that the system is adapted for different applications.
More particularly, the clear coat systems exhibit dual cross links of ester and urethane which give the system enhanced advantages over clear coats exhibiting only one type of cross linking.
Clear coats have become increasingly popular as coatings for painted articles such as automobiles. Not only do clear coats generally have excellent gloss, but they also generally offer enhanced resistance to etching and scratching. While clear coats are desirable, those employing polyisocyanates have presented certain problems in that they are sensitive to moisture. In an attempt to address many of the problems associated with employing polyisocyanates in clear coat systems, U.S. Pat. No. 4,650,718 discloses a cross-linkable coating composition based on polyepoxides and polyacid curing agents. While the resulting compositions are purported to produce cured coatings with excellent adhesion, gloss and the ability to reflect images, they generally fail to provide good humidity and mar resistance. In an attempt to address these concerns, U.S Pat. No. 4,927,868 discloses a curable, liquid coating composition containing an organic solvent and a resinous binder comprising a polyepoxide and a copolymer of an alpha-olefin or cyclolefin and an olefinically unsaturated monoanhydride which may include a partial ester thereof and preferably a polyacid.
In contrast to the present invention, it appears that the compositions of the above-mentioned patents exhibit significant secondary hydroxyl formation during the acid-epoxy reaction which, in turn, may result in the perceived humidity failures. The final thermoset polymer, i.e. clear coat composition, formed under the present invention will have both ester and urethane cross links which will synergistically enhance the properties of each other. As a result, higher equivalent weight epoxides, isocyanates and acid cross linkers can be utilized under the present invention which will generally result in significant cost savings.
Thus, the present invention fulfills a need in the art for coating compositions and, more particularly, clear coat systems which have good mar and etch resistance, a relatively high microhardness, good shelf life stability and are relatively inexpensive to produce.
The present invention provides for resinous binders useful in clear coat compositions comprising: a) a polyepoxide; b) a polyacid containing at least two carboxyl groups per molecule; and c) a polyisocyanate, wherein said resinous binder exhibits at least ester or urethane cross-linking. The invention further relates to clear coat compositions including an organic solvent and a resinous binder comprising: a) the reaction product of a polyepoxide and a polyacid; and b) an isocyanate reacted with the product of a), wherein said resinous binder exhibits both ester and urethane cross-linking. The invention also provides a process for applying a clear coat composition to a substrate wherein the dual cross-linkable system described above is in the clear coat.
The resinous binders of the clear coat compositions of the present invention are preferably obtained by the reaction of an epoxide with a polyacid to obtain a hydroxy ester which is subsequently reacted with an isocyanate in accordance with the following reaction mechanism: 
The resulting resinous binder thereby allows for extensive cross-linking including at least ester or urethane cross-linking sites, and preferably both.
The essential components of the dual cross-linkable compositions of the present invention are the reaction product of a polyepoxide and a polyacid, and the polyisocyanate.
Among the polyepoxides which can be used are epoxy-containing acrylic polymers such as copolymers of an ethylenically unsaturated monomer having at least one epoxy group and at least one ethylenically unsaturated monomer which is free from epoxy groups. In general, ethylenically unsaturated monomers containing epoxy groups are those including 1,2 epoxy groups such as glycidyl acrylates, glycidyl methacrylates and alkyl glycidyl ethers, among others.
Examples of ethylenically unsaturated monomers which do not contain epoxy groups are alkyl esters of acrylic and methacrylic acid containing from 1 to 20 atoms in the alkyl group. Specific examples of these acrylates and methacrylates are methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate.
Examples of other copolymerizable ethylenically unsaturated monomers are vinyl aromatic compounds such as styrene and vinyl toluene; nitrites such as acrylonitrile and methacrylonitrile; vinyl and vinylidene halides such as vinyl chloride and vinylidene fluoride and vinyl esters such as vinyl acetate. Acid group-containing copolymerizable ethylenically unsaturated monomers such as acrylic and methacrylic acid are preferably not used because of the possible reactivity of the epoxy and acid group.
The epoxy group-containing ethylenically unsaturated monomer(s) is preferably used in amounts of from about 5.0 to 60.0, more preferably from 20.0 to 50.0 percent by weight of the total monomers used in preparing the epoxy-containing acrylic polymer. Of the remaining polymerizable ethylenically unsaturated monomers, preferably from 95.0 to 40.0 percent, more preferably from 80.0 to 50.0 percent by weight of the total monomers are the alkyl esters of acrylic and methacrylic acid.
In preparing the epoxy-containing acrylic polymer, the epoxide functional monomers and the other ethylenically unsaturated monomers can be mixed and reacted by conventional free radical initiated organic solution polymerization.
The epoxy-containing acrylic polymer typically has a number average molecular weight between about 1,000 and 30,000, preferably 2,000 to 20,000, more preferably 2,000 to 10,000 with an epoxy equivalent weight of typically from 50-2,000, preferably 100-1,000, and more preferably 200-600. The molecular weight is determined by gel permeation chromatography using a polystyrene standard.
In determining molecular weights in this fashion, it is not the actual molecular weights which are measured but an indication of the molecular weight as compared to polystyrene. The values which are obtained are commonly referred to as polystyrene numbers. However, for the purposes of this invention, they are referred to as molecular weights.
The epoxy condensation polymers which are useful are polyepoxides, i.e., those having a 1,2-epoxy equivalency greater than 1 and up to about 3.0. Examples of such epoxides are polyglycidyl ethers of polyhydric phenols and of aliphatic alcohols. These polyepoxides can be produced by etherification of the polyhydric phenol or aliphatic alcohol with an epihalohydrin such as epichlorohydrin in the presence of alkali.
Examples of suitable polyphenols are 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 1,1-bis(4-hydroxyphenyl)ethane and 2-methyl-1, 1-bis(4-hydroxyphenyl)propane. Examples of suitable aliphatic alcohols are ethylene glycol, diethylene glycol, 1,2-propylene glycol and 1,4-butylene glycol. Also, cycloaliphatic polyols such as 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,2-bis(hydroxymethyl)cyclohexane and hydrogenated bisphenol A can also be used.
Besides the epoxy-containing polymers described above, certain polyepoxide monomers and oligomers can also be used. Examples of these materials are those containing the cyclohexane oxide moiety. These polyepoxides are of relatively low molecular weight and of relatively high reactivity, thus, enabling the formation of high solids coating compositions with excellent cure response. The polyepoxides should have an average 1,2-epoxy equivalency of greater than one. The preferred polyepoxides are diepoxides, that is, having a 1,2-epoxy equivalency of two.
Various polyepoxides containing the cyclohexane oxide moiety are known. Particularly preferred in this regard is 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate. Also, the diepoxide bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate can be used. These epoxides are commercially available from Union Carbide Corporation as ERL 4221 and ERL 4299, respectively. Also, epoxies containing the cyclohexane moiety are described in U.S. Pat. Nos. 2,890,194; 2,890,195; 2,890,196; 2,890,197; 2,890,210, 3,023,174 and 3,027,357.
The polyepoxide is typically present in the liquid crosslinkable composition in amounts of about 20.0 to 60.0, preferably from 30.0 to 40.0 percent by weight based on total weight of resinous binder.
The polyacids which are useful in accordance with the teachings of the present invention include two or more acid groups per molecule which are reactive with the polyepoxide. Preferably, the polyacid is a dibasic, tribasic or polybasic carboxylic acid functional compound. The polyacid can generally be aliphatic, cycloaliphatic or aromatic.
For example, and without intending to be limiting, the aliphatic carboxyl compounds useful in accordance with the teachings of the present invention have the general formula: HOOCxe2x80x94(CH2)Nxe2x80x94COOH, wherein n=1-20. Suitable acids include melonic, succinic, adipic, azelaic and sebacic acids, for example.
The cycloaliphatic compounds considered useful in accordance with the teachings of the present invention generally include cycloaliphatic groups having five and/or six membered rings which may be fully saturated or include some degree of unsaturation. Examples include cyclohexane dicarboxylic acid, tetrahydrophthalic acid, alkylhexahydrophthalic acid and dimer fatty acids, among others.
Useful aromatic acids include phthalic acid and isomers, and alkylated phthalic acids, in a non-limiting manner.
Half ester carboxylic acids formed by reacting a polyol with anhydrides can be used. Acrylic polymers having carboxylic acids (obtained by reacting ethylenically substituted acids and other unsaturated monomers) can also be used provided there are more than two acid groups per molecule.
The polycarboxylic acid is typically present in the cross-linkable composition in amounts of about 5.0 to 40.0, preferably from 20.0 to 30.0 percent by weight based on the total weight of the resinous binder. The ratio of epoxy to polyacid is preferably 0.8-1, based on the ratio of epoxy to carboxyl functionality being 0.8:1 to 1:1.
Among the numerous polyisocyanates, otherwise referred to herein as organic isocyanates, which are considered useful are those including aromatic, aliphatic, and cycloaliphatic polyisocyanates and combinations thereof. Examples of such isocyanates may found at columns 8 and 9 of U.S. Pat. No. 4,690,956, herein incorporated by reference.
Representative polyisocyanates are the diisocyanates such as m-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4- and 2,6-toluene diisocyanate, hexamethylene diisocyanate, tetramethylene dilsocyanate, cyclohexane-1,4 diisocyanate, hexahydrotoluene diisocyanate (and isomers), naphthalene-1,5-diisocyanate, 1-methoxyphenyl-2,4-diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenyl diisocyanate, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-biphenyl diisocyanate and 3,3xe2x80x2-dimethyidiphenylmethane-4,4xe2x80x2-diisocyanate; the triisocyanates such as 4,4xe2x80x2, 4xe2x80x3-triphenylmethane triisocyanate, and toluene 2,4,6-triisocyanate; and the tetraisocyanates such as 4,4xe2x80x2-dimethyidiphenylmethane-2,2xe2x80x2-5,5xe2x80x2-tetraisocyanate and polymeric polyisocyanates such as polymethylene polyphenylene polyisocyanate, and mixtures thereof.
Acrylic polymers having polyisocyanates (obtained by reacting ethylenically substituted isocyanates, like TMI, with other unsaturated monomers) can also be used provided there are more than two isocyanate groups per molecule.
In order to increase the shelf life of the said coating composition, and also to make certain that the acidxe2x80x94epoxy reaction takes place prior to isocyanate reaction, blocked isocyanates of the above-mentioned polyisocyanates can also be used. Examples of blocking agents are methyl-ethyl ketoximes, various alcohols and caprolactones, with an unblocking temperature in the baking range of these coatings. xe2x80x9cPreferably the isocyanate is present in amounts between about 20 to about 30% by weight based on the weight of the resinous binder.xe2x80x9d
Optional ingredients in the dual cross-linkable composition include those which are well known in the art such as surfactants, flow control agents, thixotropic agents, fillers, anti-gassing agents, organic co-solvents and catalysts, among others. Examples of such materials as well as suitable amounts of each can be found in U.S. Pat. Nos. 4,220,679; 4,403,003 and 4,147,679 which are hereby incorporated by reference.
The resulting compositions can be applied to the desired substrates by any conventional coating techniques such as brushing, dipping, flowing or, most preferably, spraying. Conventional spray techniques and equipment for air spraying, airless spraying and electrostatic spraying in either manual or automatic methods can be used.
Without intending to be limiting, the following examples are provided to illustrate many of the aforementioned concepts. As will be understood by those skilled in the art, it is preferable that clear coats generally have high acid etch resistance, have a high gloss, high crosslinking as evidenced by MEK double rub values of 150+ according to ASTM D5402 test methods, high hardness of 150+ as measured utilizing a Fisherscope(copyright) Microhardness Tester, Model NO. H100V, and good scratch resistance.